This is what we have done the last few years as airfares from Belize are so high. We do a Multi city search on kayak.com. Aifare for our RT cost about $800 from BZE.Or U.S. to Belize RT.
If we go one way either direction leaving from Cancun, or arriving in Cancun then arriving or leaving from Belize City the airfare is about $475. So you only have to do the Cancun thing once instead of R.T.
So a one way trip to or from Cancun is $7.00 to Playa, Hotel Caribbean directly behind bus station is $20. Bus to Chetumal $18.00 very nice comfortable bus, air, movies etc.5 hr ride. Water Taxi leaves Chetumal at 3.30 pm for $37.50 if you contact them direct $50 otherwise and that will take you into Caye caulker same day.
That would make the R.T about $570..with less time loss. And a mini tour of the Yucatan...